javascript basic question

While declaring Javascript in a html document. We have 3 ways to do that

1. script section goes in the head tag
2. 开发者_开发问答script section goes in the body tag
3. javascript is references from an external file

Which one of these is faster and efficient with respect to performance is considered?

Thanks

Put javascripts/references to linked scripts at bottom of page if possible. As suggested here: http://developer.yahoo.com/performance/rules.html

1 and 2 are about tag location. 3 could apply to both 1 and 2.

In addition, you can have javascript in event handler attributes, like so:

<button onclick="alert(1)">pressme</button>

to top it off, you can also have javascript as url, in for example links:

<a href="javascript: alert(1)">click me</a>

Just sticking to your examples: first of all, it is usually a good idea to use external script files that you load with a src component in tags. This allows the browser to cache the script, which allows the page to load faster after the initial page load. this is especially true if you use things like jQuery and load them from a public Conent delivery network (like the google ajax api see: http://code.google.com/apis/ajaxlibs/documentation/)

As for the location (head or body): in the olden days, people used to say, put your scripts in the head to ensure they are loaded once the body is loaded and all the interactive elements can be used by the user. But the problem with that is that the loading of those scripts will block loading of the visual part of the page, the body. Basically, the users are looking at a blank page, wondering whuy their page is taking so long to render. So nowadays, the popular wisdom is to put all scripts as far down in the body as you can, and make sure that you write your javascript in a way that it can handle partially loaded scripts. The YSlow guide is a great resource to learn about these things. see: http://developer.yahoo.com/yslow/help/

It really depends on what type of JavaScript you are writing. If you writing code that needs to be executed in the body (for ex: document.write()) you will have to write that in the body tag. If that is not the case and if you are writing javascript functions then should go in the head tag or in a different file. You would use a different file if you are going to use the same functions across many pages.

w.r.t performance, it again depends on what you are doing. If you have just one page that uses javascript, it would be faster to keep it in the header. This way you would reduce a round trip to get the javascript file.

If you have multiple pages that use the same functions, then it will be faster if the functions are in a different file because they will be downloaded once and used multiple times.

I'd say it depends on the circumstances.

• An external file is a good idea if you have a large script that is used across a site and you want to take advantage of client-side caching mechanisms.

• If a script is only used on one page then keeping it in the head/body might make sense. Clearly the earlier the script comes in the page the sooner the JavaScript will be executed, but you may be constrained by waiting for the DOM to be available to the script anyway, in which case it won't make any difference if it's in the head or the body.

• You could put the script immediately after any HTML that defines the DOM that it needs access to. This would probably be the quickest way of getting a script to execute in the page, but don't go for this over an externally loaded (and cached) file if it is large or used in many places.

If you're super concerned about performance, I would say loading the js in the html would be fastest. Items in the load before the rest of the page, so from a user's perspective, they may think the download takes longer with js since the page won't start to render until after the is loaded, but the amount of data should be the same. External js file is likely the slowest since it will require a separate http request.

The short answer is...well...it depends.

If by

faster and efficient with respect to performance is considered

you mean "loads faster", then inline script in the head will get your code into the browser faster the first time it's loaded. An external file can be cached, so if you're including the same script in multiple pages, once you overcome the overhead of loading it the first time, then you have it resident in memory.

I prefer to have my Javascript and HTML all in the same file just because I don't like having a lot of different tabs open. But then again it is a lot neater to use your Javascript, HTML, and CSS all in different files.

So far, the more complicated/long the code is, I'd use a different file, but if the code is simple, I'd just use the same file for everything.